(1) Field of the Invention
This invention relates to the processing of hydrocarbon resins. More particularly, this invention relates to the processing of hydrocarbon resins in melt-mixing mechanisms to obtain a pelletized form.
(2) Description of Related Art Including Information Disclosed Under 37 CFR 1.97-1.98
Many hydrocarbon resins such as piperylene- and dicyclopentadiene-based resins, as examples, may have low melt viscosity at elevated temperatures, excessive brittleness below their glass transition temperature, Tg, and other characteristics that make them difficult to process, as well as difficult to store, ship and/or handle. For example, it is difficult to extrude and pelletize low-melt-viscosity, brittle materials such as hydrocarbon resins because the molten strands from the die face have low strength and become brittle and break easily as they cool and solidify. Such hydrocarbon resins are normally pastillated using a rotoforming process.
It is sometimes desired to chemically modify the hydrocarbon resins, e.g., as in U.S. Pat. No. 7,276,557, which is difficult to do in the rotoforming process. Thus, the chemical modification may be carried out downstream in a post-rotoforming process, greatly increasing the cost and complexity of modification, particularly in view of the processing, storing, shipping and handling difficulties noted above.
In an underwater pelletizing system, a material, usually a thermoplastic polymer, is fed into an extruder or an extruder and melt pump which forces the molten polymer through a screen changer or diverter valve and through a die plate. As the polymer emerges from the die, pellets are cut by rotating blades and solidified in the process water flowing in a closed loop across the die face inside the cutting chamber. The process water transports the pellets to a centrifugal dryer where water is removed and the dry pellets are discharged.
Polymer materials with a low melt viscosity are notoriously difficult to process in underwater pelletizer systems, tend to smear at the die face and/or are comminuted to fines or dust before they can be packaged, e.g., while being transported in the slurry or dried in the rotary drier. See, for example, U.S. Pat. No. 5,962,033 (col. 2, 11. 56-63) and U.S. Pat. No. 5,593,702.
One solution in the case of low-melt-viscosity, brittle materials such as hydrocarbon resins has been to process the material by employing a polymeric carrier such as polyolefin. See U.S. Patent Publication No. 2009/076212. The polyolefin is mixed with the resin and thus serves to increase the melt viscosity and avoid low-temperature brittleness. While this solution is applicable to an extruder, including in an underwater pelletization process, it has the disadvantage of introducing the carrier polymer into the resin, which increases the volume and weight of the resin product and additionally the polymer may be undesirable in the end products made using the resin, e.g., in the case of thermoset polymers such as rubbers.
There is thus a need for a method for pelletizing a hydrocarbon resin, particularly a neat hydrocarbon resin without an added polymer carrier. There is also a need for a method by which the hydrocarbon resin can be reactively modified and pelletized in a single operation or unit process, without the need for post-rotoforming or post-pelletization processing to achieve the desired chemical modification.